1. Field of the Invention
The present invention relates to an ethylene-mono-alkenyl aromatic compound copolymer having good processing properties, a process for the production thereof, and a molded article thereof.
2. Description of the Related Arts
In general, an elastic recoverability, a transparency and a mechanical strength found in a polyvinyl chloride are required for a copolymer to be used as various films such as a stretch film, a shrink film, a wrap film and the like. However, the polyvinyl chloride comes in question concerning environmental pollution because of the possibility of generation of harmful substances at incineration, etc. Various kinds of polymers are presently considered as a replacement for the polyvinyl chloride, but it is the status quo that none of them have been obtained that are satisfactory from the viewpoints of viscoelastic properties and transparency.
In recent years, progress has occurred that in the field of polymerization of olefins such as ethylene and propylene. Particularly, a polymer having characteristics different from conventional polymers can be produced, and an extremely small amount of catalyst can produce a large amount of polymer, because of the use of catalysts using a transition metal compound such as a so-called metallocene, a non-metallocene or the like.
An application of such a catalyst to the copolymerization of ethylene with an alkenyl aromatic hydrocarbon represented by styrene has been proposed. For example, a pseudo-random copolymer (a random copolymer wherein methine carbon atoms with which a phenyl group is bonded are mutually separated by 2 or more of methylene groups without fail) of ethylene with styrene obtained by using a so-called homogeneous Ziegler-Natta catalyst using a specific transition metal compound and an organoaluminum compound is described in Japanese Patent Publication No.2623070. Further, an ethylene-styrene copolymer obtained by using a similar catalyst in which the arrangement of phenyl groups is sterically regulated is described in Japanese Patent Publication (Kokai) No.9-309925. Either of these copolymers is excellent in viscoelastic properties and transparency, and is expected as a replacement for polyvinyl chloride. However, there have been problems in that a copolymer with a high styrene composition is particularly insufficient in mechanical strength because the molecular weight of a polymer tends to be deteriorated in accordance with the enhancement of copolymerization composition of styrene. On the other hand, a process for copolymerizing ethylene, styrene and 1,5-hexadiene is described in J. Polym. Sci., Part A: Polym. Chem., Vol.35(1997), p2549. However, there have been problems that the copolymer obtained by this process is insufficient in the transparency and the mechanical strength because a molecular weight distribution (Mw/Mn) represented by a ratio of a weight average molecular weight(Mw) to a number average molecular weight(Mn) is extremely wide as 14.5.
Further, Japanese Patent Publication No.2623070 is known as an ethylene-alkenyl aromatic compound copolymer in which an alkenyl aromatic compound such as styrene or the like is used as a copolymer component of ethylene. However, it is hardly said that the ethylene-styrene copolymer disclosed herein has a comparatively short property relaxation time against a zero-shear melt viscosity which is judged by a melt viscosity xcex7*(0.1) and a reciprocal xcfx89c of the property relaxation time shown in literature xe2x80x9cSPE ANTEC, 1086-1090 (1997)xe2x80x9d, namely, has a low non-Newtonian property and is sufficiently excellent in processability. None of ethylene-alkenyl aromatic compound copolymer sufficiently excellent in processability has been found, and therefore, the development of such resin has been intensively desired. Further, since a film, a sheet, a molded container and the like obtained. from such resin have an extremely easy processability, its practical application is intensively desired.
An object of the present invention is to provide an ethylene-mono-alkenyl aromatic compound (herein-after, sometimes referred to simply as xe2x80x9calkenyl aromatic compoundxe2x80x9d) copolymer having an excellent processability.
Another object of the present invention is to provide a copolymer capable of taking a constitution in which a halogen is not contained having a high molecular weight and a narrow molecular weight distribution, and being excellent in an elastic recoverability and a transparency.
Further, another object of the present invention is to provide a process for producing said copolymer at an extremely high polymerization activity.
Still further, another object of the present invention is to provide a molded article comprising said copolymer and being transparent and excellent in a mechanical strength, a flexibility and an elastic recoverability.
Other objects of the present invention will be apparent from the following descriptions.
The present inventors have extensively studied the mechanism of manifesting the processability of an ethylene-alkenyl aromatic compound copolymer for a long time, and as a result, the present inventors have found that an ethylene-alkenyl aromatic compound copolymer having an improved balance between a zero-shear melt viscosity and a property relaxation time which cannot be conventionally found, a process for the production thereof, and a use thereof can attain the objects of the present invention within the range of a melt viscosity and a molecular weight distribution, and have thereby completed the present invention.
According to the present invention, an ethylene-alkenyl aromatic compound copolymer having a melt viscosity xcex7*100 at a shear rate of 100 radian/second at 190xc2x0 C. within a range of 3.0xc3x97103 to 3.0xc3x97106 poise, a molecular weight distribution in terms of ratio of weight average molecular weight(Mw) to number average molecular weight(Mn) [Mw/Mn] within a range of 1.2 to 4.0, and wherein a zero-shear melt viscosity xcex7*0 (poise) at 190xc2x0 C. and a characteristic relaxation time xcfx840 (second) which satisfy the relation of the expression (1) described below, is provided.
log xcfx840 greater than log xcex7*0xe2x88x926.38xe2x80x83xe2x80x83(1)
Further, according to the present invention, a process for producing the fore-mentioned ethylene-alkenyl aromatic compound copolymer which is produced in the presence of a catalyst using a specific transition metal compound having a group which has a cyclopentadienyl type anion skeleton, is provided.
Further, according to the present invention, a molded article prepared from the above-mentioned ethylene-alkenyl aromatic compound copolymer, is provided.